Sergio G. Sousa

Context . The origin of the lower edge of the Neptunian desert remains an open question. The leading hypotheses are tidal disruption of volatile-rich sub-Neptunes following high-eccentricity tidal migration (HEM) and atmospheric escape of their H ₂ /He envelopes. Aims . We aimed to confirm and characterise two TESS candidates near the lower edge of the desert, TOI-426.01 ( P _orb ≈ 1.32 d) and TOI-1839.01 ( P _orb ≈ 1.42 d), orbiting the solar-type stars TOI-426 ( V = 10.1 mag, G2 V) and TOI-1839 ( V = 11.0 mag, G9 V). Methods . We acquired 77 HARPS radial velocities (RVs) of TOI-426 and 70 of TOI-1839, and we performed a joint Bayesian analysis of the RVs and TESS photometry. The RVs of both targets show significant activity signals, which we modelled using shared-timescale and multidimensional Gaussian process frameworks to disentangle stellar variability and derive precise planetary parameters. Results . We confirm TOI-1839 b ( R _p = 2.27 ± 0.10 R _⊕ , M _p = 7.10 ± 0.78 M _⊕ ) and TOI-426 b ( R _p = 2.19 ± 0.09 R _⊕ , M _p = 6.7 ± 1.8 M _⊕ ), and we detect an additional transiting sub-Neptune, TOI-1839 c ( P _orb ≈ 4.02 d, R _p = 2.50 ± 0.13 R _⊕ , M _p = 6.0 ± 1.1 M _⊕ ), and a long-period giant planet, TOI-426 c (assuming a circular orbit, P _orb = 235.8 _−8.5 ^+9.2 d, M _p sin i = 444 ₋₁₇ ⁺¹⁸ M _⊕ ). TOI-426 b ( F ≈ 1700 F _⊕ ) and TOI-1839 b ( F ≈ 1050 F _⊕ ) are among the most highly irradiated sub-Neptunes known to date, but their bulk densities nevertheless require a significant volatile component. For the three transiting planets, our interior-structure and atmospheric-escape models yield short mass-loss timescales of ∼1–100 Myr for H ₂ /He-dominated atmospheres but much longer timescales of ∼100–1000 Gyr for H ₂ O-dominated envelopes, favouring a steam-world scenario for these planets. We also explored the population of short-period low-mass sub-Neptunes with outer long-period giant companions and found a strikingly high incidence (∼35%) at the shortest orbital periods near the desert edge, where successful HEM outcomes are expected to circularise. Conclusions . The survival of highly irradiated volatile-rich sub-Neptunes near the lower edge of the Neptunian desert is difficult to reconcile with a classical picture in which this boundary is set solely by the evaporation of primordial H ₂ /He envelopes. At the same time, the high incidence of long-period giant companions is consistent with an important role for HEM in shaping this region.

We present new CHEOPS transit observations of AU Mic b and AU Mic c obtained between June and September 2024, extending the baseline of transit-timing measurements of this young planetary system. For AU Mic b, the timing signal is well established, with a semi-amplitude (10 ± 3 min) and a characteristic modulation timescale (1168 ± 20 d) consistent with previous determinations. By contrast, the new CHEOPS data show that the large transit-timing deviation of AU Mic c reported previously was not sustained. After the steadily increasing timing trend observed in 2022 and 2023, the 2024 timings returned closer to the zero point of the observed-minus-calculated diagram, indicating a reversal of the previously reported behavior. For AU Mic c, both the transit-timing semi-amplitude (46 ± 26 min) and the characteristic modulation timescale (2150 ± 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

Young planets offer a unique window into the early stages of planetary evolution. AU Mic is one of the nearest (9.8 pc) pre-main-sequence stars (∼20 Myr), hosting two transiting Neptune-sized planets and a debris disk. Previous studies have shown that the rotations of the central star, the debris disk, and the inner planet b are all aligned, suggesting that the system has not undergone violent evolution. Here we report new Rossiter–McLaughlin measurements for both AU Mic b and c, which happened to transit back-to-back on 2024 August 24 and 25, using the Magellan Planet Finder Spectrograph, together with contemporaneous photometry from LCOGT and CHEOPS. We confirm the aligned orbit of AU Mic b ( λ _b = 1 ^∘ ± 12 ^∘ ) and find two possible solutions for AU Mic c: we slightly favor an aligned solution ( λ _c = −10 ^∘ ± 16 ^∘ ) but cannot rule out a polar solution ( λ c = 8 7 ∘ − 2 9 ∘ + 3 6 ∘ ). Broader considerations, including dynamical stability and transit possibility, also support the mutually aligned scenario. An unexpected stellar signal during ingress and the poor transit-timing variation predictions of AU Mic c prevent a precise constraint on its obliquity, and various attempts using chromatic spectral analyses fail to outperform simple data exclusion in mitigating stellar contamination. Our observation highlights the importance of understanding stellar activity across multiple timescales and channels when characterizing young, active systems. A robust solution for the AU Mic architecture will require either a better understanding of stellar activity or future observations fortuitously free from strong stellar contamination.

Context . Elemental abundances of FGK stars can be derived routinely from high-resolution optical spectra, but this remains considerably more difficult for cooler stars. Even estimates of the chemical composition of planet-host stars are central for studies of planet formation and interior composition, however. Machine-learning methods offer a practical route for inferring otherwise inaccessible abundances from more widely available stellar data. Aims . We investigate how much information on stellar metallicity and selected elemental abundances is encoded in kinematic and orbital properties, and whether these empirical relations can be used to estimate abundances and abundance ratios relevant for the characterisation of exoplanet hosts. Methods . We used a large APOGEE DR17 sample of red giant stars as the main training set and an independent HARPS sample of nearby FGK dwarfs for the external validation. From astrometry and radial velocities, we derived Galactic velocities, orbital parameters, and stellar population-membership probabilities. We benchmarked several machine-learning regressors, optimised the strongest models, and analysed the feature importance using gain-based metrics, permutation importance, single-feature models, and SHapley Additive exPlanations (SHAP) values. We also explored the prediction for C and O from Mg, Si, and [Fe/H] and derived simple empirical relations between selected abundance ratios (Fe/Si, Mg/Si, C/O, and Fe/O) and metallicity. Results . Kinematic information alone recovers only a limited fraction of the variance in stellar metallicity, with a clear performance ceiling at Root Mean Square Error (RMSE) ∼0.20 dex. The most informative predictor is the maximum vertical orbital excursion, Z _max , followed by radial orbital parameters. When [Fe/H] is combined with kinematic information, the abundances of C, O, Mg, and Si are predicted significantly more accurately than with the baseline approximation [X / H] = [Fe / H]. In contrast, when C and O are predicted from Mg, Si, and [Fe/H], most of the predictive power is already contained in the elemental abundances themselves, with Mg being the dominant contributor, and the addition of kinematic information provides little improvement. The trained models reproduced the main abundance trends associated with Galactic chemical evolution. We found that the slopes of the relations between Fe/Si, Mg/Si, C/O, and Fe/O and metallicity differ slightly between the HARPS and APOGEE samples, with fractional differences generally below 17%. Conclusions . Stellar kinematics and orbital parameters contain only limited information on stellar metallicity, and the predictive performance is primarily constrained by the intrinsic information content of the input features and not by the choice of the model architecture. Nevertheless, when combined with metallicity, these parameters enable accurate predictions of elemental abundances, including those that are difficult to determine directly. This provides a practical framework for extending chemical characterisation to stars for which detailed abundance determinations are challenging.

HAT-P-26,b is a well-characterized Neptunian exoplanet ̊m p R_⊕) orbiting a K1V star. It lies within the Neptunian ridge (P_ ̊m orb days), following a moderately eccentric orbit (e and hosting a water-rich atmosphere. The possible presence of a stellar companion hinted at by near-infrared (NIR) spectral emission, along with candidate planetary companions inferred from transit timing variations, calls for further investigations of the system architecture. Hence, the HAT-P-26 system is a prominent target for investigating the atmospheric and dynamical evolution of exo-Neptunes and, in particular, via the study carried out by the Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges and Savanna (ATREIDES) collaboration. We combined three transit observations of HAT-P-26,b using ESPRESSO on ESO's VLT to precisely characterize its orbital architecture and to investigate whether it has a misaligned orbit, as reported in cases of other evaporating and eccentric planets in the ridge. We employed the workflow to perform a careful and robust data reduction. We explored the system's orbital architecture using the in-built Rossiter-McLaughlin Revolutions (RMR) technique. ANTARESS HAT-P-26,b exhibits a polar orbit (projected obliquity angle of łambda = -78_ -13 ^ +13 ^ ̧irc and a 3D true obliquity angle of ψ = 85_ -5 ^ +5 ^̧irc). This places it alongside GJ,436,b, HAT-P-11,b, and GJ,3470,b as polar-orbit and eccentric planets lying within the ridge population. It has the lowest density (0.40 ) of these four planets, placing it near the density brink where full atmospheric erosion after early disk-driven migration or tidal disruption after high-eccentricity migration has been proposed to occur. Our dynamical analysis suggests that the observed polar orbit might either be a transient state arising from orbital-plane precession or a relic of high-eccentricity migration via Von Zeipel–Lidov–Kozai cycles driven by an inclined stellar companion. We note that in the latter case, the existence of planet,c would be strongly disfavored. g cm^ -3 HAT-P-26,b stands out as a special target in the Neptunian ridge, owing to its low density and polar orbit. The current dynamical analysis is limited by the uncertainties surrounding companion(s) in this system. As better constraints on the companion(s) become available, it will be particularly interesting to further explore the origin of this fluffy ridge Neptune through a joint analysis of migration, evaporation, tidal disruption, and inflation. Our results also demonstrate the importance of homogeneous data reduction when combining multi-epoch transit observations for accurate spin-orbit characterization.

The K2-138 system consists of a K-dwarf transited by six planets in the sub-Neptune regime, with radii ranging from 1.5 to 3.4 R_⊕ and orbital periods of between 2.3 and 42 days. The five innermost planets form a chain of zero-order three-body resonances, while the outermost planet is suspected to be linked to the chain through a first-order three-body resonances. The transit timing variations induced by the resonant configuration enable a precise determination of the system's architecture. The fine-tuning and fragility of K2-138 and other resonant chains ensure that no significant scattering or collision event has taken place since the formation and migration of the planets in the protoplanetary disc, and hence provide important anchors for planet formation models. We aim to improve the characterisation of the architecture of this system, in particular the planetary masses and its resonant configuration. In addition, we place this system in context with other resonant chains, and expand on the recent finding that resonant sub-Neptunes are puffier than non-resonant ones by deriving a mass-radius relationship for each population. We performed a global analysis of all available photometry and radial velocity data for K2-138 using photo-dynamical modelling of the light curve. We also tried different sets of priors on the masses and eccentricity, as well as different stellar activity models, to study their effects on the masses estimated by transit timing variations and radial velocities. We show that our joint photo-dynamical and radial velocity analysis resulted in a robust mass determination for planets b to f, with a precision of ∼ 15% for the mass of planet f, and better than 10% for planets b to e. We also show that K2-138 is indeed currently locked in the resonant configuration, the five inner planets librating around an equilibrium of the chain; however, existing data are not sufficient to verify if the outer triplet is librating in a first-order three-body resonance. Finally, we demonstrate that the mass-radius relationship of resonant sub-Neptunes is well described by M=2.54,R^0.84, which is less dense than the non-resonant population, for which we found M=2.78,R^1.32. We also found that the resonant population has a smaller spread around its mass-radius relationship than the non-resonant population, pointing towards an inter-system similarity in the composition of the resonant sub-Neptunes.

Small planets are extremely common in the Galaxy, including planets with masses and radii between those of Earth and Neptune. Characterising the mass of such planets requires ultra-precise radial velocities. The ESPRESSO spectrograph was designed and built for this purpose. We present an overview of the ESPRESSO Guaranteed Time Observations transit follow-up sub-programme, which aimed to confirm and characterise small transiting planet candidates from the K2 and TESS space missions. We analysed the global stellar and planetary properties of the sample of 65 planets in 30 systems characterised by this sub-programme. This includes six systems presented in this paper, for which we either obtain only the upper mass limits or provide updates to previously published parameters. We also placed this sample in the context of the overall population of precisely characterised small planets. After separating the population into insolation regimes, we find a tentative mass threshold at for the rocky to volatile-rich composition transition in the medium-insolation regime, along with a population of likely stripped massive rocky planets in the high-insolation regime. Likewise, we find a correlation between planet mass and stellar metallicity, with more massive planets hosted by more metal-rich stars. We also explored the radius valley, finding that planets below the gap have a tighter mass distribution. We compared planetary masses with typical protoplanetary disk masses, drawing tentative conclusions about likely formation conditions. We also discuss the impact of our observing strategy on our results. The ESPRESSO transit follow-up sub-programme has been highly productive in characterising a diverse population of small planets, which has enabled us to identify population-level features. Likewise, the lessons learned from this sub-programme will be valuable for PLATO follow-up planning.